Hydrocarbon drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information may comprise characteristics of the earth formations traversed by the borehole, along with data relating to the size, configuration, and direction of the borehole itself. The collection of information relating to conditions downhole is termed “logging.”
Drillers often log the borehole during the drilling process, thereby eliminating the necessity of removing or “tripping” the drilling assembly to insert a wireline logging tool to collect the data. Data collection during drilling also enables the driller to make accurate modifications or corrections as needed to steer the well or optimize drilling performance while minimizing down time. Techniques for measuring conditions downhole, including the movement and location of the drilling assembly contemporaneously with the drilling of the well, have come to be known as “measurement-while-drilling” techniques, or “MWD”. Similar techniques, concentrating more on the measurement of formation parameters, have been referred to as “logging-while-drilling” techniques, or “LWD”. While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably.
In logging systems, sensors in the drill string measure the desired drilling parameters and formation characteristics. While drilling is in progress, data associated with the logging system may be continuously or intermittently sent to the surface by some form of telemetry. Many logging systems use the drilling fluid (or mud) in the drill string as the information carrier, and are thus referred to as mud pulse telemetry systems. However, data transfer rates in mud pulse telemetry systems are relatively low, on the order of five bits per second or less of actual downhole data. Pulses in drilling fluid experience frequency dispersion with increasing distance traveled, and thus for extended reach of wells (e.g., between 30,000 and 50,000 feet in length) even lower data rates through drilling fluid may be experienced. Moreover, pulses in the drilling fluid do not propagate well past some devices in the drill string, such as mud motors and agitators, further limiting the applicability of mud pulse telemetry as a carrier medium of communication along the drill string.